Differential Equations
by Hedgerose
Summary: Stiles has all the variables, but he doesn't even know that there needs to be an equation yet.


**Author's Notes:** Can I warn for Kate Argent? Implied abuse of power, dubcon/noncon, age difference (15/21). Characters may perceive themselves or other characters differently from how they may be perceived from an objective viewpoint. Some aspects of this fic have been jossed, as it was written after 2x03. Thanks to my betas for their help!

* * *

Stiles has all the variables, but he doesn't even know that there needs to be an equation yet.

It's simple math, really. Twenty-one minus six is fifteen, and fifteen-year-old boys are easily swayed.

* * *

When Derek is fifteen, Katie DaSilva is his substitute bio teacher for a week, and she smells _fascinating_. There's something in her that smells like danger, like the knife-edge of adrenaline and sparks off a flint and steel, the first hint of a bottle of whiskey. She smells like a lot of different things- metal and wood and some kind of oil he isn't familiar with. She smells like the crackle of lightning on the forefront of a storm, and right now, when he's fifteen, she is everything he wants.

Katie DaSilva has gorgeous golden hair and a laugh that's warm and welcoming. She's an engaging substitute and for one short week, pretty much everyone actually _likes _biology.

Katie DaSilva thinks that he's funny and _seriously _smart; after she finishes her week with the rest of the class, she suggests that she give him private tutoring at her house three days a week, because she wants him to be the best he can be. He tells his parents he's studying at the library (and they do meet there, the first day, so they don't hear him lying), because even the first time, there's something a little strange about it.

Katie DaSilva kisses him for the first time after they've been working together for two weeks, and in the moment her lips touch his (it's not his first kiss, not even remotely, but he can still count the people he's kissed on one hand and she rounds it out) this is exactly what he wants.

She just kisses him once that first day. "For a job well-done," she says, handing him the paper she'd had one of her former college advisors review, just before she leans in and gets closer than she's ever been.

The paper has three slight corrections and _excellent _written at the top in red ball-point pen.

It's one of the things he loses in the fire.

* * *

They're trapped in the woods, on the run from _something_- neither of them are quite sure what- when the scent of Stiles' fear gets tinged with arousal for the first time. The _thing _passes them and Stiles goes back to all-terror-all-the-time; Derek puts his arm across Stiles' chest and holds him against a tree until it's gone. They walk back to the car as quietly as they can, dry leaves and branches cracking under their feet far too often for Derek's comfort.

"What _was _that?" asks Stiles once they're more or less safely ensconced in his Jeep.

"No clue," Derek says, shrugging. There's an openness in Stiles right now, and Derek feels an answering tug, pulling him towards Stiles instead of pushing him away, and in that moment Derek _wants_.

He wants Stiles in all of his awkward teenage glory, the contrast of his thin wrists and broad hands, the way he trips over himself and the sharpness of his mind. He doesn't know if it's the wolf who wants Stiles or if it's just him; he doesn't know if there's really any difference between the two.

He'd had a few significant others in college, girlfriends and boyfriends, people he'd found interesting first and desirable later. Stiles reminds him of Brian, with his close-cropped hair and too-loud, gregarious temperament; Stiles reminds him of Helena's quick mind and quicker tongue.

They don't trust each other fully, he and Stiles. Derek _needs_ a pack- any alpha does- and he's looking for other people who are like he is, kids who are just a little broken, just a little cracked- kids for whom the pack can be a real family, not just a crowd to run with, hunt with. He keeps Isaac safe and he makes Erica healthy; he gives Boyd people to _belong_ with. But Stiles doesn't see that- he sees Derek taking his classmates, his almost-friends, people he sees in the cafeteria and in his classes, and making them _other_. And Derek is doing that, sure- he's making them _better_. They are stronger as a pack than any of them are individually, and Stiles will see that, someday.

(He doesn't offer the bite to Stiles and he probably never will because Stiles doesn't _need _that same way- Stiles has his dad and Scott and Scott's mother, a whole network of support. Stiles' eyes get bleak sometimes, but it's not the same thing at all.)

But none of that changes the fact that there is this spark, this unsettlingly mutual thread of _want_ between them, and when Derek gets back to the tiny apartment he's renting (under an assumed name; he pays the rent once a week in cash), he brings his hands to his face to search for one last hint of _Stiles _still lingering on them.

Then he sheds his clothes, leaves them in a heap in the entryway, and stands in the shower, washing his hands over and over until the waters is cold, because he _cannot_ do this.

* * *

There are a thousand possibilities but only one reality, and the reality is this: Stiles is sixteen, and Derek is twenty-three.

* * *

"You smell wrong," Laura announces. She hooks her chin over his shoulder and wraps herself around him, getting as close as she physically can to him. "What's scaring you?"

"I'm not scared," he lies, dry-mouthed and knowing she'll hear his heart beating faster, hear the difference in his tone. (Katie (_call me Katie- Mrs. DaSilva is my mother_) had touched him that same way the night before, wrapped herself around him from the back, but his chest was bare then and so was hers. He shoves that down and doesn't think about it, doesn't think about the way Katie looked in the half-light of her bedroom or the way she'd touched his chest).

"You are," Laura argues. "Derek, you're freaking _terrified_."

He shrugs. "It's nothing," he tries.

Laura sighs and rubs her face against the side of his neck, marking him as _hers_- she's still a beta right now, but they both know that when things shift around, she'll be his alpha, so he's happy to be hers. She's his sister, and he loves her, and more importantly, he _trusts _her. He trusts her not to push right now, not when he's still figuring all this out and trying to weigh the fact that his bio grade is through the roof and he's got an offer at a lab to work this summer against the way that Katie makes him feel.

When he's with Katie it's like he's a different person. His eyelids feel heavy as she licks up his belly, as she holds herself over him and teases him until his hips jerk up of their own accord and she slaps him across the face, laughing.

He feels his face smile at her and his hands come up to cup her face; she takes one of them and leads it lower, lower-

There's nothing on her face, no expression at all, and it startles him out of this weird haze of sex and heat, but then she's smiling again, leaning down to kiss him just this side of too hard and he gets lost again in it all. Sometimes, Katie looks at him and he can't read her at _all_; he wonders what she sees him as. He wonders if she sees _him _at all, and in those moments he wonders if he really exists.

He is hers sometimes more than he is Laura's, certainly more than he is his mother's. His mother is the alpha right now, but she's pushing him away because that is what alphas do. She wants him to be his own person and not cling to the pack quite so much, because she says that sometimes he leans on them too much, cleaves too tightly to her, Uncle Peter, or Laura.

If he can't be his own and if he could choose from any of them, he'd like to be Laura's.

* * *

Lydia figures it out first- of course she does.

"You know that Stiles has a thing for you," she says bluntly, like she's expecting it to shock him.

"I know," he says. He's been trying to ignore it, the way that Stiles' scent and body language change when he's around Derek. Heat rises in Stiles' cheeks when he talks to Derek and his speech is just a touch faster. When they'd first met that was all down to fear and intimidation; if Stiles was a wolf, his tail would've been entirely between his legs for those first few tense months. Instead, he talked too fast and tried to handle things entirely outside of his league. He follows Scott around like a puppy and somehow gets brought under Derek's tentative umbrella of _pack_.

Somehow in all of that the fear and intimidation dissipates; Derek borrows Stiles' clothes and does _not_ take them home and roll around on them to return them smelling like himself. He lets Stiles drive him around. He and Stiles grow into this- _thing_, where Stiles isn't nearly as intimidated of Derek as he pretend to be and Derek is a great deal moreso. They're not quite friends, and neither of them trusts the other one hundred percent, but what they have is enough.

Stiles is the only one who touches him without intending to hurt him (even if it's just to straighten his jacket, even if it's just a hand on his arm); maybe that's what hurts the most.

* * *

The wolf thing is less of a problem than it should be.

The thing is that Derek's always been a wolf. He doesn't _know_ any different; he doesn't know how to be anything other than half-human. He fakes it well enough- he knows how to dress himself, he knows how to cook and clean and all those other stupid human tricks. He lived in _New York _for years, without anything larger than Central Park to run around in (and that'd been taken decades ago by the Rizolli pack; Derek didn't have nearly the seniority to ask for rights, and he hadn't asked Laura to do it for him), so being back in California with acres of forest and meadow to run and leap in is like- it's like taking a breath of fresh air, being able to stretch fully after years of sleeping in a closet.

And at the same time it's like the air is full of smoke every time his mind wanders. He chokes at the scent of a barbecue and can't stomach the thought of cooking his own meat for weeks after he comes back. He has blood under his fingernails and dirt caked on his boots; he buries his sister as a wolf, plants the flowers to make her stay that way.

He takes three deep breaths and thinks about the clean California air, the crunch of a rabbit in his jaw, the itch of wolfsbane growing so close (close enough to the house that he won't ever be able to forget his sister). He doesn't cry, not then, not with his sister under fresh dirt and no ideas about who could have done this (the Argents, the Argents, it could be Katie- it could be _Kate_- but he hears from Joshua who hears it from Emersen that Kate Argent was in Idaho when his sister was cut in half).

Derek doesn't think of the wolf as a separate animal. Its needs and desires are close enough to his that he doesn't make some dividing line between human and animal, mind and beast. The wolf likes to hunt rabbits; the human is happy to eat them, once they're cooked. The wolf smells everything and reacts, positively or negatively, and Derek does the same thing because _they _are the same.

So the wolf smells _desire_ on Stiles. It smells _arousal_ and it smells _fear_; the combination makes him sick to his stomach, because Laura says- _said-_ it's what he smelled like just before the fire (when he was meeting Katie DaSilva for private tutoring three days a week- _Derek, you're so talented in biology, I could show you so much_- and she'd left perfume on his skin and her hands on his shoulders, his chest). He doesn't want to be that for someone else. He _can't _be that for someone else.

He starts avoiding Stiles more than he should, if he wants to maintain proper pack dynamics. He hopes some distance will put things in perspective for Stiles, make him realize that there is no way for them to be together in a healthy and productive way. Stiles is too young to really know himself, and Derek is old enough to know that all too well.

* * *

It's simple math: Stiles is sixteen and Derek is twenty-three.

Sixteen-year-olds are just as easily swayed as fifteen-year-olds; Derek keeps his hands off Stiles' shoulders and doesn't lean in too close.

* * *

"It's different in all the ways that matter," Lydia argues.

"It really isn't," Derek snaps, and he walks away, because he doesn't care how she knows about him and Kate, about him and Stiles; he can't finish this conversation any other way. She is his beta, and she should know her place, but since he's trying to keep his pack together and not send it splintering he lets her go.

(He touches Erica like Kate used to touch him because she needs to believe she can be beautiful and powerful and all the things he can see inside her; afterwards he is sick in a hospital bathroom and can barely hold his shaking hands together to rinse out his mouth, because she is seventeen and as much as he needs a pack this is not the way he wanted to build one, out of broken pieces that even the wolf can't hold together. There is too much of Kate in him, too much of Peter, but some days fighting against them is just too much.)

* * *

The parts of him that are all wolf love Stiles. They want to curl up next to him at night. More than anything, they want to _play_. When he's all wolf around Stiles, his tail drops until it shows that he's as relaxed as he can get; he smiles wolfy smiles and sometimes (embarrassingly) actually _wags his tail_.

He brings Stiles a dead possum and after Stiles finishes making disgusted faces (and _noises_; Derek can usually understand what people are saying when he's all wolf, but Stiles' tone helps) he throws it back into the forest as far as it will go. Before he knows it, Derek is bounding off after it, like they're playing _fetch_ and he's a _dog,_not a wolf. He stops himself twenty feet from where he knows the possum landed and considers if he actually wants to be this undignified.

The others are far enough away that no one will believe Stiles when he'll inevitably try to tell them about playing fetch with Derek and a dead possum.

He'll just have to make sure that he dunks himself in the creek a time or three to wash off dead-possum and live-Stiles, or else they'll know that Stiles is telling the truth.

The wolf doesn't want to mate with Stiles; there's enough human in the wolf to recognize that Stiles is still a juvenile, that he's not really a suitable candidate for a werewolf's mate. The wolf cares and it doesn't. It wants to be close to Stiles, and it likes the idea of holding him down and licking him until he smells like _them_, like _pack_, but it's little enough that Derek can push it down, sublimate it under years of telling his mind and body _no._

* * *

Katie calls him one morning to cancel tutoring; it's the first time she's called him at home instead of on his cell.

"I'm sorry," she says. "I have other plans."

"It's fine," he responds, feeling his shoulders relax. "I have a lit project to work on, anyway."

"Great! Well, I'll see you on Thursday," she says. "My place?"

"Yeah, that'd be- yeah. Dad's working from home all week on some project, so your place'd be better," he says, and she laughs her big, warm laugh.

"Thank you, sweetie," she says, and she hangs up on him.

Laura's looking at him when he hangs up the phone. "Who was that?" she asks, voice carefully neutral.

"Friend from bio," he says, willing his heart rate to stay the same, for the prick of nervous sweat on his palms to remain out of Laura's notice.

"I wish you'd just _tell_ me," she says, and he ducks his head, avoiding her gaze. It's stupid and embarrassing, because isn't this every guy's dream? To have the hot blonde substitute _all over you_? Who _wouldn't _want that? Is there something wrong with him because he would really rather never see Katie DaSilva again?

She catches his arm on his way out the door. "You would tell me, right?" she asks. "If something was wrong- even if you didn't want to go to mom or dad or Uncle Peter, you know you could tell me, right?"

"I know," he says. "I'll- I'm fine, Laura."

"You're really not," she says, like it's news, like he doesn't feel his hands shake when Katie calls, like he could get out of this if he wanted to. "Half the time, you smell like someone else, someone that I don't know, and if I can tell that you smell so wrong, I _know _mom and dad can- I don't know why they haven't-"

"_Please_," he says, quietly. Really, he's been dying to tell someone, and he doesn't have any friends at school who will understand. "I'll tell you after school," he offers.

"Promise?"

"I do- I promise, okay?" He looks down again; he can't meet her eyes, not after lying to her for so long.

"Hey, c'mere," she says, and she pulls him in for a hug, rubbing the side of her face against his. "You're still mine, okay? You're still one of us, even when you smell like someone else."

He leans in against her and closes his eyes, wishing for a too-long moment that it were true.

That afternoon they're barely out of the high school when Laura starts to shake and twitch. Derek has just long enough to wonder what's wrong before he feels it, too- the fire snaking up the line of his ribs, smoke choking his lungs and making it absolutely impossible to breathe. They cling together on the front steps of Beacon High until a teacher finds them and leads them back inside. Someone tries to call their parents but the call doesn't go anywhere, and that's when the administration figures out something is wrong.

No one can explain how Derek and Laura knew about the fire that was happening miles away in the woods, but it's brushed off as one of those weird things about the Hales and forgotten.

Laura forgets to ask, in the aftermath of everything, in the shock of being a new alpha and taking care of Uncle Peter and Cousin Jen (who Peter had pulled out of the fire but the doctors are saying things like _make it easier _and Derek and Laura are too heavy with the knowledge that Jennie isn't going to survive long enough to see Uncle Peter wake up).

Kate Argent who is Katie DaSilva leaves him a note in his locker wrapped around a sprig of wolfsbane, thanking him for all his help. He doesn't get it until weeks after the funerals, because he doesn't go back to school until it's safe for Uncle Peter to be out of the hospital, until Laura feels confident enough in herself to be the alpha of their pack of two.

He's only back at school because he's cleaning out his locker; Laura wants them to move to New York, almost as far away from Beacon Hills as you can get without leaving the country.

The note is sealed with wax and he breaks it without thinking, and it smells like the graveyard, like the wolfsbane he and Laura had planted above the graves of their mother, their sisters, cousins, aunts.

Laura finds him an hour later, leaning against the lockers with the note still crumpled in his hand. He can't let go of it, not yet, but she peels away his fingers one by one and takes it from him. She reads it and her mouth thins into a firm line; he waits for her to tell him to leave, for her to blame him for the fire (it's all his fault, he's known this, but now he has proof), for her to take her claws and tear out his throat because it's what he deserves.

She reaches for him and he tilts his head up, baring his throat for her, and she freezes, arms out-stretched. "Jesus _fuck_, Derek," she breathes. "I'm not gonna- not in the middle of the _hallway_."

"Right, of course," he says quietly, and he looks down. Of course not, they're in public, there's no way she would, but when they get _home_ (not home anymore; it's an efficiency apartment just to sleep in until they move and it's not _home_ it'll never be _home_ because home doesn't exist any more- his fucking _bio tutor burned it to the ground_-).

Laura's arms are around him and the note is crumpled up even further but it's the first time he's felt safe since the fire. Maybe she's not furious with him. He buries his face in her neck and keens softly until everything stops hurting quite so much. Laura just holds him and lets her own tears sink into the cotton of his t-shirt. They don't talk about it, they _never _talk about it, but that's how Laura finds out about Kate.

She still loves him, but once they're in New York she wants to meet every friend, every study partner and their parents, and Derek brings them over, gladly, because every time she sees someone new it makes him feel another mile farther away from California.

* * *

_Kate Argent gets dressed in the scattered pieces of Derek Hale's clothing and lies in bed until she's sure she smells like him and not like a human at all._

_Then, she goes hunting._

* * *

The one thing he didn't expect is Stiles actually coming to talk about it with him.

He just _appears _out of the dusk one night, walking up to what remains of Derek's house like he's a frequent guest, hands loose in his pockets. Derek is sitting on the front steps waiting for him; he'd heard Stiles' Jeep miles away and decided that waiting and avoidance really weren't doing anything to fix this, but maybe a direct conversation would. He just doesn't expect Stiles to be the one to bring it up first.

"Hey," Stiles says.

Derek just tips his head up in greeting.

Stiles shrugs and chuckles. He walks over to the steps and sits down a few steps below and directly in front of Derek, then leans and tilts his head back. "You're tall," he states, and it startles Derek into an ungraceful laugh.

"You're an idiot," he says.

"Aw, c'mon," Stiles says. "Don't be like that. You know you love me."

There's a pause like an indrawn breath, tense and anticipatory; Derek knows what Stiles is asking and that he's made it a statement so that he can laugh it off, pretend it was a joke, but Derek doesn't know quite how to respond, because it wasn't a joke, not really.

"Stiles-" he starts, but he's let the silence go on too long, and Stiles is already up and moving.

"Forget it," Stiles says, striding back towards his Jeep on still-growing legs.

"Wait," he says, and Stiles actually does stop for a moment, before he shakes himself and keeps walking. Derek pushes himself off the porch and moves faster than Stiles can and catches his shoulder.

Stiles spins around to face him. "What?" he says, sharp.

"It's not-" he says, stops. He still doesn't know how to say this. "You're part of my pack," Derek explains.

"Lydia said." Stiles isn't looking at him, and it seems so _wrong_, because Stiles always looks back; Stiles is the one who stands up to him. Stiles is the one who isn't as intimidated as he pretends to be. "She said that she could, ah, smell me a mile off when I'm around you, I thought you knew-" His face is bright red and he is _beyond _embarrassed.

"I know," Derek confirms. "It's not exactly subtle."

And that's the wrong thing to say, clearly, because Stiles' nostrils flare and he's hurt and angry and Derek just wants to fix it. "It's fine," Derek says. "I don't mind- I know that-"

"Whatever," Stiles says, cutting Derek off, and he's walking away again. This time he stops himself and whirls around. "You know, for a long time I thought you were just ignoring me and that you thought I was stupid. But then I almost cut your arm off and there was the whole thing with your uncle- I thought that you, y'know, thought I was okay. Maybe."

"You're fine, Stiles. It's not a problem."

"It's not a _problem_? Okay, fine, whatever. It's not like I actually expected you to return my clearly infantile and idiotic feelings, but you could at least throw me a bone, here- and I did _not _mean that literally, just so you know."

"Stiles, you're sixteen," Derek says.

"I _know_," Stiles says through clenched teeth. "I am painfully aware of this fact, okay? And I'm glad that you have basic memorization skills, seriously. What does that have to do with anything? I know how old you are, I know what I'm- what I _want _to get myself into, okay?"

"I'm twenty-three."

"Also aware of _that _fact, thank you. Don't you know that older guys are hot?" He pauses, looks at anything but Derek. "Lydia says that sometimes you smell like you want me."

Derek feels his stomach drop and his hands grow cold; he is suddenly so glad that Stiles isn't a werewolf, that he can't smell Derek's trepidation and his guilt. "If you were older-"

"Oh come on, that's such bullshit," Stiles scoffs.

"It's really not," Derek says mildly. If he can just stay _calm_ through this, maybe he can come out the other side with most of his dignity and his relationship with his pack intact. "There's a huge gap in _everything_ between us, Stiles, and-"

"And I'm the most mature teenager you know?" Stiles jokes, looking momentarily hopeful. Derek's mouth twitches but he refuses to let himself actually smile, even though the constant joke-making is one of the things about Stiles that he actually appreciates, for all that it's annoying half the time.

"You're the most mature _kid _I know," Derek counters.

"Ouch," Stiles says. "Clearly you are far wiser than I am, old man of _twenty-three_."

Derek thinks somewhat fondly back on the days when all he had to do to shut Stiles up was threaten him. "It's not just the fact that I am _seven years _older than you," Derek explains. "It's- Kate Argent and I used to-"

"Used to what? Be friends? Chem lab partners? Play croquet together?"

"No." Derek says. "She was my bio tutor for six weeks when I was fifteen." Even as he's saying it he knows that it won't be enough. Stiles is the sheriff's son and he'll want to get to the bottom of this; Derek won't be able to dig himself out.

"And?"

"And she needed access to my parents' house, what the fuck do you _think_ she did?" Derek snarls, getting in too close to Stiles' face, willing him to have pushed far enough, to be satisfied with this and draw his own goddamn conclusions. He's never actually _told _anyone- Laura found out, and she's the only person he could have imagined telling. He sits back on the stairs and waits for Stiles to figure it out.

"I'm guessing she didn't bribe you with doggie treats," Stiles says, voice rising enough to make it a question and not a statement.

"No," Derek says. He looks at the tops of the trees over Stiles' head, ignoring the boy standing in front of him and the burnt rafters behind. "She put on the clothes I had left in her bedroom and burned my family alive."

He glances at Stiles long enough to see him putting the pieces together- _her bedroom_ and _I was fifteen _and Derek's emphasis on the fact that there are seven years between the two of them. He sees Stiles' cheeks turn white, then red, watches as he ducks his head, ashamed at having asked.

Derek never says _she took advantage of me_ because it sounds like a bad romance novel; he doesn't say _I don't think I was in love with her but sometimes it felt like I was close_. Stiles has enough of the pieces- he doesn't need to see the whole sordid picture. Stiles is seven years younger than Derek is and they aren't even friends, not really.

If things were different, Derek might give them a chance. If he was twenty-eight and Stiles was twenty-one; if they had met when Stiles was in college and Derek was working at a coffee shop or as Stiles' TA or any one of a million possibilities. But this is the reality: Derek is damaged goods, and Stiles would drown with him, pulled down by Derek's cracks and the weight of his guilt. Derek would wreck Stiles, leave him a ruin- and what's worse is that Derek might be _happy _about it, might let the ghosts of Kate and Peter out to play too much.

Stiles has been silent for too long. Derek stares at the trees and tries not to think about anything other than the changing sky and the leaves that fall slowly to the forest floor. He feels Stiles settle in next to him, too close and still too far away.

"I'm sorry," Stiles says quietly. "How old was she?"

"Twenty-one," Derek murmurs, like a confession.

"Okay," Stiles says, and he leans in against Derek's side. It's just that- Stiles sharing his warmth and safety with Derek. It's a tangible sign of trust, and Derek stiffens immediately, drawing up straight, scenting Stiles' apprehension and unease. But Stiles stays there, a silent presence, until Derek relaxes inch by inch.

"The answer is still no," Derek says, and Stiles tips his head until it touches Derek's shoulder.

Stiles shrugs. "I know," he says. Derek sighs and closes his eyes for a moment, then carefully, slowly, wraps an arm around Stiles' shoulders.

Together, they wait for the sun to go down.

* * *

It's simple math: Stiles is sixteen and Derek is twenty-three.


End file.
